1. Field of the Invention
The present invention relates to a material for the negative electrode of a lithium secondary battery.
2. Related Background Arts
Lithium secondary batteries having organic electrolysis solutions have been widely used. Their advantage is the high energy output per unit volume or unit weight in comparison with other batteries. In exploiting this advantage, researchers and engineers have been advancing the development and practical applications of the batteries as power sources for mobile communication devices, notebook-type personal computers, and electric cars.
In order to improve the performance of a lithium secondary battery, attempts have been made to use metallic lithium as the negative electrode. However, the repetition of charge and discharge causes dendritic metallic lithium to grow on the surface of the negative electrode. This may lead to an internal short circuit between the negative and positive electrodes, ultimately triggering explosion. To avoid this hazardous situation, engineers have been studying the formation of an inorganic solid electrolytic layer as the electrolyte on the metallic lithium. They have also been studying the formation of a protective layer to prevent the surface deterioration of the inorganic solid electrolytic layer.
However, it remains to be found how the thickness of the inorganic solid electrolytic layer and the protective layer is to be selected in order to obtain a lithium secondary battery having high charge-discharge efficiency and an excellent charge-discharge-cycle performance.
If the inorganic solid electrolytic layer is excessively thick, the prolonged film-formation time increases the production cost, and bending or other external force exerted during the battery-manufacturing process tends to generate large cracks in the layer.
An object of the present invention is to offer negative electrode that provides excellent performance to a lithium secondary battery.
The present invention offers a lithium-secondary-battery negative electrode that comprises metallic lithium or a lithium-containing metal, a first inorganic solid electrolytic layer formed on the metal, and a second inorganic solid electrolytic layer formed as a protective layer on the first inorganic solid electrolytic layer. In the foregoing negative electrode, when the ratio of the thickness b of the second inorganic solid electrolytic layer to the thickness a of the first inorganic solid electrolytic layer b/a is more than 0.5, a lithium secondary battery that has an excellent charge-discharge-cycle performance can be obtained. It is more desirable that the ratio b/a be 1 or more.
However, if the second inorganic solid electrolytic layer is excessively thick, although the effect of coating is enhanced, the ionic conductivity decreases and the amount of the second inorganic solid electrolytic layer that dissolves in the electrolyte increases, thereby degrading the performance of the battery. Therefore, it is desirable that the foregoing ratio b/a be less than 2.
It is desirable that the thickness a of the first inorganic solid electrolytic layer be less than 1 xcexcm. The thickness less than 1 xcexcm can reduce the production cost and suppress the generation of large cracks when bending or other external force is exerted on the layer during the battery-manufacturing process.
The first inorganic solid electrolytic layer may comprise a sulfide-containing lithium-ion-conductive compound, desirably a sulfide-containing lithium-ion-conductive compound that contains at least one member selected from the group consisting of Li3PO4, Li4SiO4, and Li2SO4.
It is desirable that the second inorganic solid electrolytic layer comprise a constituent contained in the first inorganic solid electrolytic layer except sulfide. This condition enables a satisfactory second inorganic solid electrolytic layer to grow on the first inorganic solid electrolytic layer. The second inorganic solid electrolytic layer may comprise a lithium compound containing at least one member selected from the group consisting of Li3PO4, Li4SiO4, and Li2SO4.
The first and second inorganic solid electrolytic layers may be amorphous.
The present invention enables the production of a highly stable lithium secondary battery having high energy density and an excellent charge-discharge-cycle performance.